Conventionally, a carbon material is generally used as a negative electrode of a lithium ion secondary battery. Recent years, it has been often reported that such a lithium ion secondary battery has produced abnormal heat and ignition (so-called thermal runaway). It is considered that an internal short circuit of the battery is one of causes of the thermal runaway described above. When the internal short circuit of the battery occurs, excessive inrush current flows to the negative electrode and causes heating of the negative electrode or other members.
As a cause of the internal short circuit, besides external impact, there is considered a breakage of a separator by a column-like metal lithium crystal (dendrite) deposited on a surface of the negative electrode. In a lithium ion secondary battery using a carbon material as the negative electrode, the above-mentioned metal lithium crystal is apt to be deposited because the potential of the negative electrode is low like 0.08 V (vs. Li).
On the other hand, it is also known to use spinel type lithium titanate (S-LTO) besides the above-mentioned carbon material as the negative electrode of the lithium ion secondary battery. In the lithium ion secondary battery using this negative electrode, the potential of the negative electrode is raised to 1.55 V (vs. Li), and hence the metal lithium crystal is hardly deposited on a surface of the negative electrode. As a result, risk of the internal short circuit can be reduced, but there is a drawback that the negative electrode geometric capacity is only 175 mAh/g (carbon material has 372 mAh/g).
In addition, it is also proposed to use a bronze-type titanium oxide compound as the negative electrode of the lithium ion secondary battery (see, for example, Patent Document 1). In the lithium ion secondary battery using this negative electrode, the potential of the negative electrode is raised close to 1.5 V (vs. Li) similarly to a case of using S-LTO, and hence the metal lithium crystal is hardly deposited on a surface of the negative electrode. As a result, risk of the internal short circuit can be reduced, and in addition, the negative electrode geometric capacity can be increased to 335 mAh/g compared with the case of using S-LTO.